Fault detection devices employing eddy currents generated in a workpiece surface are known in the art and include a probe with one or more inductive coils. An electromagnetic signal is provided to the coils. As the coils are moved relative to the workpiece surface, a small fault or defect (e.g. a crack) located therein will cause a change in the magnetic flux density. This change will be reflected as a perturbation in the impedance characteristics of the electrical circuit of which the coils are a part. The electrical circuit is configured so that the magnitude of a perturbation of an electrical signal therein is correlated to the magnitude of the defect.
An example of such an eddy current instrument is the Nortec NDT 25. The Nortec instrument is capable of flaw detection in a workpiece surface or it can measure the thickness of non-conductive coatings such as paints. The Nortec NDT 25 is used with a probe having one or two inductive coils with a core and is designed to operate over a range of selected signal frequencies in accordance with the desired range of defect size. An example of a known probe is the Pratt & Whitney TAM 189532.
Prior art eddy current detection devices require the inductive probe to be positioned at a fixed distance from the surface of the workpiece. Very accurate probe-workpiece spacing must be maintained since variations in probe distance from the workpiece surface will alter the output signal magnitude; rendering accurate assessment of flaw size (or even presence) impossible. Eddy current flaw detection devices have either been one of two types. Contact devices are configured with an outer surface of the probe that remains in contact with the workpiece surface throughout a scan. Other devices have the probe positioned off the workpiece surface by an intermediate element, such as an air bearing. Contact devices are undesirable since the contact surface of the probe will wear in a very short time so that the distance will vary by an indeterminate amount. In addition, the rate of wear of the probe contact surface increases with the speed at which the probe is moved across the workpiece surface. The faster the measurement, the more quickly the probe will wear. Eddy current flaw detection devices using air bearings or the like are undesirable, sure they are mechanically complicated and are liable to fail.
The workpieces themselves have varying surface contours. Initially the probe is positioned at a selected distance from a point on the workpiece surface. As it moves thereacross, the separation will vary. Often workpieces may assume a "tin can" or "potato chip" surface configuration. That is, the workpiece may bow inwardly or outwardly in a manner similar to a metal can or assume a saddle shape akin to that of a potato chip. With many devices the variation will be of a magnitude greater than the probe-workpiece spacing, resulting in contact therebetween and damage to the probe.
It would be advantageous to have an eddy current surface flaw detection method and apparatus for detecting workpiece surface flaws which would compensate for inductive probe-surface height variations. The present invention is drawn towards such a method and apparatus.